In order to understand features of the present invention, an electric power unit adopting a conventional higher harmonics control method will be described with reference to FIGS. 19 and 20. FIG. 19 is a circuit diagram of an example of a conventionally used electric power unit. An electric power unit 100 in FIG. 19 is constituted of an alternating current power source 101 which is, for example, a commercial power source of 100V 50 Hz to general household, a bridge-type rectifier 102 for rectifying the alternating current power source 101, a first reactor 103, a diode 104, a second reactor 105, a capacitor 106 and a smoothing capacitor 107. The bridge-type rectifier 102 consists of four diodes D101, D102, D103, D104 in bridge connection.
The first reactor 103 has its one end connected to a + output of the bridge-type rectifier 102 and the other end connected to an anode of the diode 104. The second reactor 105 has its one end connected to the + output of the bridge-type rectifier 102 and the other end connected to one end of the capacitor 106. A cathode of the diode 104 is connected with the other end of the capacitor 106, and connected also to one end of the smoothing capacitor 107. The other end of the smoothing capacitor 107 is connected to a - output of the bridge-type rectifier 102. A load 108 of the power unit 100 is connected to both ends of the smoothing capacitor 107.
Now, the operation of the electric power unit 100 will be described. FIG. 20 is a diagram of waveforms of a half cycle in the electric power unit 100 shown in FIG. 19. An input voltage Vin from the alternating current power source 101 is a sine wave as indicated in FIG. 20. A current I101 running in the first reactor 103 shows a bent waveform because a charging current to the smoothing capacitor 107 is smoothed at the first reactor 103. A current I102 running in the second reactor 105 shows a waveform oscillating with a resonant frequency of the second reactor 105 and capacitor 106. An input current Iin becomes a sum of I101 and I102, assuming a waveform as shown in FIG. 20. Since the waveform of the input power is softened and a peak value of the current is lowered as above, higher harmonics are suppressed further.
The aforementioned arrangement has drawbacks, though. As described hereinabove, in the prior art, such current is supplied that supplements higher harmonics with the utilization of the resonance of the second reactor 105 and capacitor 106 and consequently, the resonant frequency is required to be set at three times a frequency of the power source. For example, when the power source has a frequency of 50 Hz, the resonant frequency should be set to approximately 150 Hz. The second reactor 105 should be approximately 10 mH supposing that the capacitor 106 has a capacity of 100 .mu.F. In the conventional arrangement, both the second reactor 105 and the capacitor 106 should have large capacities in order to suppress the higher harmonics. The first reactor 103 alike should have a large current capacity to cope with a large current in case of a large load. From these reasons, the conventional electric power unit is disadvantageously bulky in size and is expensive.